Human evolution and the obstetrical dilemma: The pelvic floor hypothesis.

Human childbirth is mechanically difficult because a large-headed, broad-shouldered fetus must pass through a comparatively narrow, twisted bony birth canal. Traditional explanations of this "obstetrical dilemma" emphasize the role of bipedal locomotion in inhibiting the evolution of a wider, more spacious pelvis. Yet recent evidence suggests that additional selective pressures shaped human pelvic anatomy. The pelvic floor hypothesis proposes that expansion of the bony birth canal would compromise the pelvic floor's stability by increasing the distance over which it must span to support the lower abdominal organs. Here, we summarize comparative, clinical, and population-based empirical evidence, as well as findings from biomechanical modeling studies, in order to evaluate the pelvic floor hypothesis. Across mammals, pelvic canal shape and size covary with posture and gravitational loading. In humans, wider and more mediolaterally expanded pelves are consistently associated with an increased risk of pelvic floor disorders, while biomechanical models show disproportionate increases in pelvic floor deformation and tissue stress with increasing canal width. Evidence that pelvic floor disorders can reduce biological fitness supports their evolutionary relevance. Together, these findings identify pelvic floor stability as an important and previously underappreciated factor constraining the evolution of the human pelvis.